The invention relates in general to a device for temperature control, which can be used to heat components coupled thereto to temperatures of up to 1500° C., to maintain these at this temperature or to cool these back to room temperature, and in particular to a device in which the heat transfer to and from a component can be designed to be variable.
In the course of the exploration of materials that must satisfy high thermal requirements during use, the need arises to test these objects under adequate conditions in advance. This results a need for a device that is able to maintain an object or a sample at a constant temperature between room temperature and approximately 1500° C.
In the course of research conducted on the plasma-wall interaction (PWI) in fusion reactors, for example, different materials are examined with regard to the behavior thereof when exposed to plasma. These primarily involve hydrogen or deuterium plasma, which is generally conducted frontally at the material sample and thus results in hydrogen inventory in the material, among other things. The quantity thereof is established using various analytical methods by releasing the inventory. For this purpose, the sample ideally has a freely selectable but constant temperature, with or without plasma exposure or during the analysis. By applying an electrical field between the sample and the plasma source (biasing), it is possible to vary the ion velocity of the plasma, and thus the impact energy and penetration depth. The desired maximum sample temperature is derived from the anticipated wall temperatures of a fusion reactor.
In particular, this results in a need for a device that is able to heat a material, or a sample of the material, to a temperature above 1500° and maintain this temperature, optionally also under vacuum.
In general, the problem of heat transfer arises in the case of components that are mounted on top of, and make planar contact with, one another, since in general continuous planar contact cannot be assumed with a direct fixed component coupling system. Rather, different, unavoidable small elevations of the surface areas are kept at a distance from one another, and a non-contact space will develop between the two surfaces. In relation to the few actual contact surfaces, this intermediate space has an extent that is quite significant and clearly unfavorable with respect to heat transfer.
This problem arises to an intensified degree at a coupling site if the components to be contacted, in the form of test objects, are repeatedly exchanged. Previously customary intermediate layers made of graphite film, for example, are able to reduce, but cannot prevent, the aforementioned effect.
Moreover, it has been found that adhesions frequently remain at the coupling site upon detachment of the component (object), which may possibly accumulate and superimpose and thus, in turn, disadvantageously reduce the actual contact surface with a further test object.
Under atmospheric conditions, the heat transfer is created by the direct contact (body contact) with another component or with air molecules (convection) and by radiation. This principle usually takes place in the range up to approximately 800 to 1000° C. Applications requiring higher temperatures are generally carried out under vacuum so as to avoid any influence on the material, such as oxidation, and thus structural changes, or ultimately combustion. In this case, however, convection is eliminated due to the lack of free air molecules. While the heat transport in the case of body contact is approximately linear, the power dissipation caused by radiation increases to the fourth power of the absolute temperature. Balanced configuration of the surfaces and controlled heat transfer thus becomes increasingly important.
Two scenarios, which typically occur during the test experiments, deserve a closer look, these being the heating and the cooling of an object in question.
If an object is to be heated, ideally the entire heat output of an adjacent heater should flow primarily into the object, preferably as a result of good contact. This becomes even more important, the higher the desired temperature is for the object. However, this also means that the heat flow of the heater should be as low as possible in other directions, which is to say in the direction of a mount, or in the direction of a cooler, for example.
The opposite applies when an object is to be cooled. As large a surface area of the object as possible would then be advantageous which, as a result of contact, can bring about appropriate heat withdrawal by way of coolers.
So as to maintain a predefined object temperature at a constant level independent from outside influences, an interplay between or a mixture of these two scenarios would thus have to be achieved. The coupling of the object to the heater and to the mount or the cooler consequently plays a crucial role.
Permanently good surface area contact of the object with the heater has the advantage of good heat input when there is a need to heat the object. In this case, thermal decoupling of the cooler to as great an extent as possible is advantageous. The cooler itself should not be shut off for its own protection, and should be maintained at the maximum temperature permissible for the material used. The lower this is, however, the greater, the resulting heat sink will, of course, be.
If cooling is needed, the heater is generally shut off (but nonetheless, due the composition thereof and/or due to the material thereof, may represent a less favorable heat conductor in relation to the surrounding component material). The thermal coupling to the object and the mount thereof should be at a maximum, so as to ensure the necessary withdrawal of heat from the object, despite disadvantageous (parasitic) marginal phenomena (heater, clamping).
The clamping mechanism disposed at the edge, which generates the pressing for the component coupling system having variable heat transfer, represents good body contact with the object. While, in the case of heating, this device has the disadvantage of acting as a heat sink, additional cooling is advantageously created on the side of the object located opposite the cooler.
When heating of objects, the heat loss to the surroundings must always be taken into consideration. Under atmospheric conditions, cooling by way of convection plays a crucial role, while under vacuum the heat loss is generally created by thermal or infrared radiation since, in this case, no, or only few, molecules are available for the heat transport due to the system. Heat transfer essentially only takes place by way of radiation or direct planar contact. At high temperatures, radiation emission grows to the fourth power of the absolute temperature.
Deliberate and controllable heat transfer would, therefore, be desirable to stabilize a desired temperature in a component, but protection of the components disposed directly adjacent thereto must also be taken into consideration. Maximizing the heat transfer is desirable to reduce the transmission losses in the energy input or energy withdrawal chain. On the otherhand, deliberate thermal insulation with respect to the mount or frame could also be advantageous.